C-X-C chemokine receptor type 4 (CXCR4), also known as fusin or cluster of differentiation 184 (CD184), is a seven transmembrane G-protein coupled receptor (GPCR) belonging to Class I GPCR or rhodopsin-like GPCR family. Under normal physiological conditions, CXCR4 carries out multiple roles and is principally expressed in the hematopoietic and immune systems. CXCR4 was initially discovered as one of the co-receptors involved in human immunodeficiency virus (HIV) cell entry. Subsequent studies showed that it is expressed in many tissues, including brain, thymus, lymphatic tissues, spleen, stomach, and small intestine, and also specific cell types such as hematopoietic stem cells (HSC), mature lymphocytes, and fibroblasts. CXCL12, previously designated SDF-la, is the only known ligand for CXCR4. CXCR4 mediates migration of stem cells during embryonic development as well as in response to injury and inflammation. Multiple roles have been demonstrated for CXCR4 in human diseases such as cellular proliferative disorders, Alzheimer's disease, HIV, rheumatoid arthritis, pulmonary fibrosis, and others. For example, expression of CXCR4 and CXCL12 have been noted in several tumor types. CXCL12 is expressed by cancer-associated fibroblast (CAFs) and is often present at high levels in the tumor microenvironment (TME). In clinical studies of a wide range of tumor types, including breast, ovarian, renal, lung, and melanoma, expression of CXCR4/CXCL12 has been associated with a poor prognosis and with an increased risk of metastasis to lymph nodes, lung, liver, and brain, which are sites of CXCL12 expression. CXCR4 is frequently expressed on melanoma cells, particularly the CD133+ population that is considered to represent melanoma stem cells; in vitro experiments and murine models have demonstrated that CXCL12 is chemotactic for such cells.
Furthermore, there is now evidence implicating the CXCL12/CXCR4 axis in contributing to the loss or lack of tumor responsiveness to angiogenesis inhibitors (also referred to as “angiogenic escape”). In animal cancer models, interference with CXCR4 function has been demonstrated to alter the TME and sensitize the tumor to immune attack by multiple mechanisms such as elimination of tumor re-vascularization and increasing the ratio of CD8+ T cells to Treg cells. These effects result in significantly decreased tumor burden and increased overall survival in xenograft, syngeneic, and transgenic cancer models. See Vanharanta et al. (2013) Nat Med 19: 50-56; Gale and McColl (1999) BioEssays 21: 17-28; Highfill et al. (2014) Sci Transl Med 6: ra67; Facciabene et al. (2011) Nature 475: 226-230.
These data underscore the significant, unmet need for CXCR4 inhibitors to treat the many diseases and conditions mediated by aberrant or undesired expression of the receptor, for example in cellular proliferative disorders.